Electro-acoustic-optical devices include an optically defractive medium capable of having a moving optical grating induced in it in response to acoustic waves propagating therein. An electro-acoustic transducer, usually mounted at one end of the medium, launches acoustic waves in the defractive medium in response to electric excitation of the transducer. Such devices are used in time integrating correlators and apparatus for determining time difference of arrival of two signals.
FIG. 1 is a schematic diagram of a prior art device for determining the time difference of arrival of signals that RF sources 10 and 12 derive. RF sources 10 and 12 are typically continuous wave sources having phases representing positions of objects being tracked. The signals that sources 10 and 12 derive are linearly combined in electronic difference circuit 14 that derives an output signal having an amplitude directly proportional to the difference between the instantaneous amplitudes of the signals sources 10 and 12 derive. The difference signal that circuit 14 derives drives piezoelectric crystal 16, bonded to one end of optically diffractive medium 17 that forms Bragg cell 18. Piezoelectric crystal 16 responds to the signal from circuit 14 to launch an acoustic wave in medium 17. The acoustic wave induces a moving optical grating in medium 17. Medium 17 is formed as an elongated cell, and crystal 16 is arranged such that acoustic waves propagate in the elongated direction of the cell. Typically medium 17 is made of gallium phosphide (GaP), which is favorably employed because it has high bandwidths of, for example, 2 GHz.
Laser source 20 derives an unmodulated coherent optical beam 22 that illuminates a center portion of cell 18. Beam 22 is incident on a first front face of cell 18 and is displaced from a line perpendicular to the propagation direction of the acoustic waves in cell 18 by the Bragg angle of the refractive material in medium 17.
Cell 18 responds to the moving optical grating crystal 16 induces in it to diffract and amplitude modulate the coherent energy in beam 22. The modulated coherent energy in beam 22 emerges from cell 18 as a series of beamlets propagating from the second, back face of the cell. The deflection angles of beamlets 24 are determined by the diffractive index of the medium 17 where the beam 22 is incident on the medium; the refractive index is determined by the amplitude of the acoustic waves propagating in the cell.
Beamlets 24 are incident on collimating lens 26 which converts the beamlets into parallel beamlets 28 which are incident on photoelectric detector array 30. Photoelectric detector array 30 includes many detector elements 32, each of which derives a separate variable amplitude output signal commensurate with the amplitude of the optical energy in the beamlet 28 incident thereon. Detector elements 32 are arranged in linear array 30 that extends in the same direction as the propagation direction of the acoustic energy in cell 18. Electric leads in bus 33 supply the signals that detector elements 32 derive to processor 34, which compares the amplitudes of the outputs of detector elements 32 to derive signals indicative of the amplitudes of the optical energy incident on each of detector elements 32 and an indication of which detector 32 has the highest amplitude optical energy incident thereon. Processor 34 responds to the amplitudes of the signals in bus 33 to derive an indication of the difference in time of arrival (i.e., the phase difference) of the signals that sources 10 and 12 derive.
We realize that a problem with the apparatus illustrated in FIG. 1 is a tendency for difference circuit 14 to combine the output signals of sources 10 and 12 in such a manner that the signal which actually drives piezoelectric crystal 16 is not exactly equal to the difference between the signals of RF sources 10 and 12. Consequently, when RF sources 10 and 12 are identical to each other and are supplied at exactly the same time, i.e., with the same phase, to difference circuit 14, the difference circuit frequently does not produce a zero output signal. Consequently, processor 34 does not derive an accurate indication of the time difference of arrival of the signals that sources 10 and 12 derive.
Bragg cell 18 has also been used in time integrating correlators which determine the time difference of arrival of RF signal sources 10 and 12. The correlator illustrated in FIG. 2 includes Bragg cell 18, responsive to a coherent optical wave that RF source 10 amplitude modulates. A moving optical grating is induced in cell 18 in response to RF source 12.
To these ends, source 10 directly amplitude modulates coherent wave beam 41 laser 40 derives. Coherent wave beam 41 is incident on diverging lens 42 which produces a diverging beam 43 incident on collimating lens 44. Lens 44 supplies collimated, coherent optical wave beam 45 to a first, input face of Bragg cell 18, which defracts the optical beam incident on it as a function of the moving optical grating induced in the cell as a result of the acoustic waves that piezoelectric crystal 16 launches in the cell. Crystal 16 responds to a signal including the variations of RF source 12, as modified by DC bias source 46 and by RF carrier source 50, typically having a frequency of about 2 GHz. Electronic adder 48 combines the RF output signal of source 12 and the DC bias of source 46 to produce an electronic sum signal that is heterodyned in mixer 52 with the RF carrier wave which source 50 derives. Mixer 52 produces an amplitude modulated electric wave having approximately a 2 GHz carrier. The output signal of mixer 52 drives crystal 16.
Bragg cell 18 responds to the optical energy in beam 45 and the acoustic wave launched by crystal 16 to produce an amplitude modulated optical beam that drives a spatial filter including focussing lens 54 and collimating lens 56, such that focussing lens 54 responds to the output beam of Bragg cell 18 and collimating lens 56 produces a collimated beam that is incident on detector elements 32 of detector array 30. Each of detector elements 32 produces an electric signal having an amplitude indicative of the optical energy incident thereon. Bus 33 supplies these signals to processor 34 which responds to them to indicate the relative time of arrival of the signals of RF sources 10 and 12 at the inputs of laser 14 and crystal 16, respectively.
A problem with the apparatus illustrated in FIG. 2, which is described in an article by Houghton et al., entitled "Spread Spectrum Signal Detection Using a Cross-Correlation Receiver," HMSO London 1995, is that it ignores transform errors between the different wave domains formed as a result of the signal from RF source 10 being transduced into an optical wave and the RF signal of source 12 being transduced into an acoustic wave.
It is, accordingly, an object of the present invention to provide a new and improved time difference of arrival detecting apparatus.
Another object of the invention is to provide a new and improved electro-acoustic-optical device.
A further object of the invention is to provide a new and improved electro-acoustic-optical device and to an apparatus for using same, wherein waves that are combined in an optical defracting medium are launched in the same wave domain.
An additional object of the invention is to provide an electro-acoustic-optical device that enables a system in which it is used to be easily callibrated.
A further object of the invention is to provide a new and improved method of calibrating a system including an electro-acoustic-optical device.